1. Field of the Invention
The present invention relates in general to the sport of archery, and in particular to determining the distance from the archer to the intended target for improved shot accuracy.
2. Description of Related Art
The capability of an archer to accurately judge the distance to his or her intended target is critical to shot accuracy. Unlike projectiles from guns, an arrow's trajectory is very non-linear over relatively short distances. In the case of hunting with a bow, incorrect compensation for target distance and the resulting arrow "drop" can lead to a missed shot or worse yet, a wounded animal.
In order to provide background information so that the invention may be completely understood and appreciated in its proper context, reference may be made to a number of prior art patents and publications as follows:
U.S. Pat. No. 4,617,741 to Bordeaux et al discloses an electronic rangefinding apparatus for attachment to a bow that allows the user to bracket a target of known size inside a transparent viewing window. This bracketing action is accomplished by electronically moving an upper bracketing line within the viewing window by actuation of a control switch. Once the object is bracketed, the device electronically estimates and displays range by relating bracket size to target distance. This general method of range estimation has been employed in a number of other patents, including U.S. Pat. No. 4,984,372 to Blizzard, U.S. Pat. No. 4,787,739 to Gregory, and U.S. Pat. No. 3,696,517 to Larson. This method is sensitive to the type and size of the target, and erroneously assumes that all targets of the same type are of the same size. Furthermore, a specific action is imparted upon the device by the archer during the ranging operation. This differs from the current invention, in which target range is estimated by sensing the angle made by an archer at a calibrated elevation as he draws the bow down and aims at the target. Range is then determined trigonometrically from said angle and elevation.
However, the concept of triangulation for rangefinding in archery has been addressed in U.S. Pat. No. 4,785,541 by Lowry wherein a device is described in which fluid level in a translucent tube is observed against a graduated scale calibrated for various heights above the ground. The device rotates when drawn down, changing the liquid level against the scale and hence indicating the range. Although this method is based on triangulation, several critical aspects are not addressed that are integral to the present invention. First, the present invention utilizes a liquid crystal digital display to numerically present the range without reading a scale. Secondly the present invention, as will be explained later in detail, electronically compensates for infinitesimal differences in height to provide improved accuracy through precise triangulation. Furthermore, the present invention determines range based on whatever measurement scale the archer is using, thus not limiting him to yards as is typically used in most rangefinding devices. This aspect is perhaps the most critical, since a significant number of archers calibrate their bow's sighting system to ranges measured by individual "paces", with actual measurements unique to the walking stride of each archer.
Triangulation is also the basis for U.S. Pat. No. 4,894,921 by Barlow wherein a device is described in which a weighted pendulum arm is attached to a rotary switch shaft to rotate the switch as the bow inclination changes. Depending upon a particular switch closure, one of several light emitting diodes or lamps will illuminate, thus indicating which sight pin should be appropriately used. The primary disadvantage of this device is that only a discreet number of switches is utilized, and thus ranges reported can only be considered approximate, with typical resolution of only plus/minus five yards. Furthermore, the device proposes to compensate for various elevations by manually changing the pendulum's position relative to the switch shaft. This method of compensating for elevation would only be accurate if the trigonometric function relating elevation to distance was linear, which it is indeed not. In the present invention, an analog signal is provided that is proportional to the non-linear trigonometric transfer function. Hence continuous resolution is attained, and by varying with infinite resolution the magnitude of the supply voltage proportional to the elevation, a true range based on the mathematic principles of triangulation is achieved.
Furthermore, several "rangefinding sights" for archery have been proposed that pivot a sighting pin relative to the angle of inclination from an elevated position or provide for manual sight pin adjustment after range has been estimated by the archer. For example see U.S. Pat. No. 4,580,349 by Webb et al. These merely lead the archer to compensate for the additional projectile drop in a longer shot by causing the bow to be aimed higher. They do not in any way determine and report the actual range.
Whatever the precise merits, features and advantages of the above cited references, none of them achieves or fulfills the purposes of the current rangefinding capabilities of the present invention.